THE EVOLUTION THEORV 



of external conditions, 

 primary characters of 



and thus is an inevitable outcome of the 

 the living substance? Or is it, though 

 primaeval in its beginnings, a phenomenon of adaptation, which 

 depends on a special mechanism, and does not occur everywhere in 

 equal extent and potency ? 



We have already become acquainted with some facts which must 



incline us to the latter view 



zckl 



Fig. 35 B (repeated). Hydra viri- 

 dis, the Green Fresliwater Polyp. 

 Section through the body-wall, 

 somewhere in the direction of ov 

 in Fig. 35 A. Eiz, the ovum lying in 

 the ectoderm (cc(), and including 

 zoochlorellas (schl) which have im- 

 migrated from the endoderm ^»j() 

 through the supporting lamella 

 {st). After Hamann. 



The globular Alga-colonies of Volvox 

 (Fig. 63) consist of two kinds of cells, 

 of which only one kind, the reproductive 

 cells, possess the power of reproduchig 

 the whole, the others, the flagellate, or, 

 as wo called them, somatic cells, being 

 only able to produce their like, but never 

 the whole. 



New investigations which have been 

 carried out by Dr. Otto Hiibuer in my 

 Institute have placed these facts beyond 

 doubt. We may conclude that, in 

 this case, a disintegration of the germ- 

 plasm has taken place during ontogeny, 

 by means of differential cell-division, so 

 that only the reproductive cells i-eceive 

 the complete germ-plasm, while the 

 somatic cells receive only the deter- 

 minants necessary to their own specific 

 differentiation, the somatic determinants. 



In this case regeneration and repro- 

 duction coincide ; thei-e is no regeneration 

 except the origin of a new individual 

 from a reproductive cell. 



Let us now ascend to the lowest of 

 the Metazoa, for instance, the freshwater 

 polyp, Hydra (Fig. 35 A), and we find a 

 high degree of regenerative capacity in 



the restricted sense, for, in addition to the 

 power of producing germ-cells, that is, cells which, when two combine 

 in amphimixis, give rise again to a new animal, almost any part of 

 the polyp can regrow a whole animal. Not only has Hydra been cut 

 in from two to twenty different pieces, but it has even been chopped 

 up into innumerable fragments, and yet each of these, under favour- 

 able circumstances, was able to grow again into a complete animal. 

 Nevertheless, we are not justified in concluding that every cell 



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